Auto merge of #18592 - MortimerGoro:dom_texture, r=jdm

Implement DOM to texture

<!-- Please describe your changes on the following line: -->

This is a prototype of the WebGL DOMToTexture feature. The API should be fine as a privileged extension for now due to security concerns. The working group has been investigating the viability of unprivileged usage but the spec is not ready yet.

Demo video: https://www.youtube.com/watch?v=hpZqEM5hPao

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18592)
<!-- Reviewable:end -->

components/servo/lib.rs

@@ -553,13 +553,18 @@ fn create_constellation(user_agent: Cow<'static, str>,
     };
 
     // Initialize WebGL Thread entry point.
-    let (webgl_threads, image_handler) = WebGLThreads::new(gl_factory,
-                                                           window_gl,
-                                                           webrender_api_sender.clone(),
-                                                           webvr_compositor.map(|c| c as Box<_>));
+    let (webgl_threads, image_handler, output_handler) = WebGLThreads::new(gl_factory,
+                                                                           window_gl,
+                                                                           webrender_api_sender.clone(),
+                                                                           webvr_compositor.map(|c| c as Box<_>));
     // Set webrender external image handler for WebGL textures
     webrender.set_external_image_handler(image_handler);
 
+    // Set DOM to texture handler, if enabled.
+    if let Some(output_handler) = output_handler {
+        webrender.set_output_image_handler(output_handler);
+    }
+
     let initial_state = InitialConstellationState {
         compositor_proxy,
         embedder_proxy,